The present invention relates generally to tunable lasers, and more particularly to a tunable laser including a plurality of vertical cavity surface emitting lasers.
Data transmission systems often transmit data over fiber optic lines using dense wavelength division multiplexing (DWDM). In DWDM a number of light signals at differing wavelengths are used to simultaneously transmit data over a fiber optic line. Conventionally, each of the light signals is generated using a laser designed to optimally transmit light at a particular wavelength. A number of unique lasers is therefore used in system operation, which increases costs complicates system build out and replenishment logistics.
One type of laser suitable for some data transmission systems are vertical cavity surface emitting lasers (VCSELs). VCSELSs may be relatively easily manufactured on a substrate, and a number of VCSELs may be manufactured, usually in an array, on a single substrate. VCSELs are made with very short cavity lengths of typically a few microns, such that only one longitudinal Fabry-Perot mode falls within the gain bandwidth of the active quantum wells. VCSELs therefore naturally form a single longitudinal mode device. Of course if the lateral size of the device is too large, or if the lateral confinement is poor, multiple transverse modes may appear. Furthermore, some form of asymmetry is generally required to lock the mode to a single polarization.
Unlike other types of lasers, such as distributed feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers, the wavelength of light emitted from a VCSEL depends simply on its cavity length. VCSELs emitting light at different wavelengths can be fabricated simply by varying the cavity length for different VCSELs. Tuning of the wavelength of emitted light is generally more complicated, however, as the cavity length generally must be electro-optically or mechanically varied. Therefore various tunable lasers have been demonstrated where the cavity length is adjusted by a micromechanical element, such as described in U.S. Pat. Nos. 5,291,502 and 5,771,253, the disclosures of which are incorporated by reference. The performance of such tunable lasers is generally compromised since small fluctuations or mechanical vibrations can cause the lasing frequency to shift.
VCSEL arrays can be made in two dimensions, and thus can be very compact. Various techniques can be used to modify the cavity length of such lasers arrays, such as described in U.S. Pat. No. 5,882,468, the disclosure of which is incorporated by reference. Unfortunately, coupling light, particularly sufficient light for use, from such arrays into a single mode fiber presents difficulties.